S3QL is a file system for online data storage. Before using S3QL, make sure to consult the full
documentation (rather than just the man pages which only briefly document the available userspace
commands).
The s3qlcp command duplicates the directory tree source-dir into dest-dir without physically copying the
file contents. Both source and destination must lie inside the same S3QL file system.
The replication will not take any additional space for the file contents. Metadata usage will increase
proportional to the amount of directory entries and inodes. Only if one of directories is modified later
on, the modified data will take additional storage space.
s3qlcp can only be called by the user that mounted the file system and (if the file system was mounted
with --allow-other or --allow-root) the root user.
After the replication, both source and target directory will still be completely ordinary directories.
You can regard <src> as a snapshot of <target> or vice versa. However, the most common usage of s3qlcp is
to regularly duplicate the same source directory, say documents, to different target directories. For a
e.g. monthly replication, the target directories would typically be named something like
documents_January for the replication in January, documents_February for the replication in February etc.
In this case it is clear that the target directories should be regarded as snapshots of the source
directory.
Exactly the same effect could be achieved by an ordinary copy program like cp-a. However, this procedure
would be orders of magnitude slower, because cp would have to read every file completely (so that S3QL
had to fetch all the data over the network from the backend) before writing them into the destination
folder (at which point S3QL would de-duplicate the data).
SnapshottingvsHardlinking
Snapshot support in S3QL is inspired by the hardlinking feature that is offered by programs like rsync or
storeBackup. These programs can create a hardlink instead of copying a file if an identical file already
exists in the backup. However, using hardlinks has disadvantages:
• backups and restores always have to be made with a special program that takes care of the hardlinking.
The backups must not be touched by any other programs (they may make changes that inadvertently affect
other hardlinked files)
• special care needs to be taken to handle files which are already hardlinked (the restore program needs
to know that the hardlink was not introduced by the backup program)
S3QL snapshots do not have these problems, and they can be used with any backup program.
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